Carbon dioxide (CO2) is a compound present as a gas in the Earth's atmosphere. It is exhaled by animals and used by plants in photosynthesis. Depending on temperature and pressure, carbon dioxide exists in one of the three phases—gas, liquid, and solid. At atmospheric pressure, carbon dioxide sublimates (changes directly from the solid phase to the gas phase) at a temperature of −109° F. (−79° C.). The energy required for this phase change is more than double the energy required to change an equal mass of water from solid to liquid at atmospheric pressure. Carbon dioxide is produced in large quantities by a variety of industrial processes, including fermentation of carbohydrates and ammonia production.
Because of its abundance, safety, and thermodynamic properties, solid (frozen) carbon dioxide is widely used as an expendable refrigerant. Solid carbon dioxide is commonly referred to as “dry ice” because it sublimates without the formation of a liquid. Dry ice is generally manufactured by first cooling and pressurizing (to about 20 Atmospheres) gaseous carbon dioxide to change the phase to liquid. The liquid is then released through an expansion valve into a cooled chamber at atmospheric pressure. The expansion further cools the carbon dioxide. About half the liquid freezes to form what is known as dry ice snow and the other half changes to gas. The gas is then recycled or released to the atmosphere. The dry ice snow is compressed in block presses to form blocks or extruded to form pellets.
Dry ice pellets are extruded in three standard diameters: one-eighth inch, one-fourth inch, and one-half inch. The pellets typically drop from the extruder directly into large insulated containers. When full, the containers are covered and shipped. A typical container has a volume of about 27 cubic feet and can hold about 1300 pounds of dry ice pellets. Although the containers are insulated, some heat transfer from the surroundings to the dry ice pellets occurs. As a result, some of the dry ice sublimates during shipping and storage. The loss of dry ice from sublimation from manufacturing to use is commonly known as the “shrink.” Purchasers of dry ice pellets typically pay the manufacturer based on the weight of delivered dry ice.
It would be highly desirable to increase the weight of dry ice pellets in a container. Any such increase would result in a direct reduction in shipping costs. It would also be highly desirable to reduce the amount of sublimation that occurs during shipping and storage, i.e., between manufacture and use. Accordingly, a demand exists for a method of packaging dry ice pellets in a shipping container that increases the quantity of pellets that fit in the container and that reduces the rate of sublimation.